This invention relates to a heat-sensitive recording material which has an overcoat layer containing a resin curable upon exposure to an ionizing radiation (hereinafter, referred to as "ionizing radiation curable resin"), and particularly to a heat-sensitive recording material which is high in gloss, excellent in color density of the recorded images and image-retainability, and further excellent in recording runnability and printability.
There has been known heat-sensitive recording materials utilizing the color forming reaction between a color former and a color developer, in which the two color forming materials are thermally contacted each other to produce recorded images. These heat-sensitive recording materials are comparatively inexpensive and can be used on a recording equipment which is compact and requires fairly easy maintenance. Because of these advantages, the heat-sensitive recording materials have been used not only as the recording medium of facsimiles and various computers, but also in the other various fields such as heat-sensitive labels. However, one major problem with heat-sensitive recording materials is their low resistance to fingerprints or solvents. If the recording layer comes in contact with a sebum of human being or a solvent, the color density of the recorded images is decreased or an unwanted coloration called "background fogging" occurs.
With a view of solving the problem, several methods have been proposed; in one method, an aqueous emulsion of a resin having film-forming properties and resistance to chemicals is coated on a heat-sensitive recording layer (Japanese Laid-Open Patent Publication No. 128,347 of 1979); and in another method, a water-soluble high molecular weight compound such as polyvinyl alcohol is coated on a heat-sensitive recording layer (Japanese Laid-Open Utility Model Publication No. 125,354 of 1981). However, the methods so far either suffer from additional problems or find themselves incapable of achieving the intended results to satisfactory levels.
For instance, in the method in which an aqueous resin coating is applied to a heat-sensitive recording layer, the temperature for drying has to be limited to a certain level in order to avoid unwanted coloration of the recording layer due to high temperature drying, whereby curing of the resin layer inevitably becomes insufficient for preventing its sticking to a recording head during recording. In order to avoid this problem, there is proposed a method in which a resin component capable of curing upon exposure to electron beams is coated on a heat-sensitive recording layer and the coated resin component is cured upon exposure to electron beams. However, the resulting heat-sensitive recording material is still unsatisfactory in terms of the image-retainability. In addition, there may also be problems that the electron beam-curable resin layer causes coloration of the heat-sensitive recording layer just after it has been coated or causes fading of the recorded images.
Under these circumstances, we, inventors, had investigated on the improvement of these difficulties and had found that, when an intermediate layer containing an aqueous resin was formed on the heat-sensitive recording layer and then an overcoat layer containing an electron beam-curable resin was formed on it, a heat-sensitive recording material which had an improved image-retainability with no fogging of the recording layer and also widely varying surface characteristics and which was superior in recording characteristics could be obtained (Japanese Patent Publication No. 42,835 of 1989). However, this proposal gave no sufficient result in the recording runnability.
Recently, the heat-sensitive recording materials have been widely used as the labels for commercial goods and, particularly in the case it is recorded in a high speed, for example, in a full-automatic labeller, an adhesion of the recording head to the overcoat layer during recording becomes a serious problem. In this case, as a method for preventing the sticking (adhesion) between the recording head and the overcoat layer, it may be considered to adding a silicone leveling agent in the overcoat layer. However, it has been found that, though the silicone leveling agent prevents sticking to some extent, it lowers printability. For example, it deteriorates the transfer of ink during printing or lowers the adhesion of ink to the recording material so that a sufficient printing effect can not be obtained.
Therefore, an object of the invention is to dissolve the above problems to provide a heat-sensitive recording material which has an overcoat layer comprising a ionizing radiation curable resin on the heat-sensitive recording layer and which is excellent in recording runnability and printability as well as in color density of the recorded images and image-retainability.